The integrity of the spine, including its subcomponents like, the vertebral bodies and intervertebral discs that are well known structural body parts forming the spine, are key to a patient's health. These parts may become crushed or damaged as a result of trauma or injury, or damaged by disease (e.g., by tumor, auto-immune disease) or as a result of wear over time or degeneration caused by the normal aging process.
In many instances, one or more damaged structural body parts can be repaired or replaced with a prosthesis or implant. For example, specific to the spine, one method of repair is to remove the damaged vertebra (in whole or in part) and/or the damaged disc (in whole or in part) and replace it with an implant or prosthesis. In some cases, it is necessary to stabilize a weakened or damaged spinal region by reducing or inhibiting mobility in the area to avoid further progression of the damage and/or to reduce or alleviate pain caused by the damage or injury. In other cases, it is desirable to join together the damaged vertebrae and/or induce healing of the vertebrae. Accordingly, an implant or prosthesis may be configured to facilitate fusion between two adjacent vertebrae. The implant or prosthesis may be placed without attachment means or fastened in position between adjacent structural body parts (e.g., adjacent vertebral bodies).
Typically, an implant or prosthesis is secured directly to a bone structure by mechanical or biological means. One manner of spine repair involves attaching a fusion implant or prosthesis to adjacent vertebra bodies using a fixation element, such as a screw. Most implants and their attachment means are configured to provide an immediate, rigid fixation of the implant to the implantation site. Unfortunately, after implantation the implants tend to subside, or settle, into the surrounding environment as the patient's weight is exerted upon the implant. In some cases, this subsidence may cause the rigidly fixed attachment means to either loosen, dislodge or potentially damage one or more of the vertebral bodies. Furthermore, after insertion into the vertebral body, the fixation element or fixation system may work itself loose and/or back out, i.e., withdraw from the vertebral body. The consequence of back out or loosening includes improper or incomplete fusion, loss of stability, potential risk to the patient, and a separate costly and often painful revision surgery.
It is therefore desirable to provide a spinal fusion implant that avoids the problem of screw loosening or back out over time and with use. In addition, it is desirable to provide an implant and associated fixation elements that can account for subsidence that occurs with the implant subsequent to implantation while also providing rigid fixation.
Although the following discussion focuses on spinal implants or prostheses, it will be appreciated that many of the principles may equally be applied to other structural body parts within a human or animal body.